A biaxially oriented crystalline polypropylene film (BOPP) has been widely used as packaging films by utilizing its excellent rigidity, transparency, moisture proof property, etc. However, since BOPP has a difficulty in the heat sealing property, BOPP has hitherto been widely used as a laminate film obtained by laminating a resin having an excellent heat sealing property on one or both surfaces thereof or co-extruding BOPP together with the resin having an excellent heat sealing property.
Hitherto, the property required for the heat-sealing resin has been to what extent the heat-sealing temperature can be decreased. That is, the low-temperature heat sealing property has hitherto been considered to be most important. This is because if the heat-sealing temperature of the heat-sealing resin can be decreased, a bag-making speed using the laminate film can be increased to improve the productivity. As the matter of course, however, the properties such as the transparency, the blocking resistance, etc., of the resin are also important.
However, recently, the diversification of packaging forms, that is, the diversification of materials to be packaged and the diversification of packaging machines due to the diversification of the materials are rapidly advancing, and a hot tack property which has not hitherto been regarded so important for BOPP has been regarded as important as the low-temperature heat sealing property.
Hitherto, various proposals have been made as the heat-sealing resins for BOPP. That is, a propylene-ethylene copolymer obtained by copolymerizing propylene and about 5% by weight of ethylene is well known as a propylene-based heat-sealing resin. Although the copolymer has excellent transparency, blocking resistance, etc., the copolymer is very insufficient in the low-temperature heat sealing property. When the content of ethylene in the copolymer is increased to improve the low-temperature heat sealing property of the propylene-ethylene copolymer, the low-temperature heat sealing property is improved to some extent but there is a problem that the transparency and the blocking resistance greatly deteriorate.
A propylene-butene-1 copolymer obtained by copolymerizing propylene and butene-1 is also well known. For example, propylene-butene-1 copolymers obtained by polymerizing in an inert solvent (so-called slurry polymerization), from which components dissolving in the inert solvent are removed, are disclosed in JP-A-50-128781 and JP-A-55-17542 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and these copolymers have a good transparency and a good blocking resistance and shows a considerably good result on the low-temperature heat sealing property.
Further, JP-A-56-22307 discloses a propylene-butene-1 copolymer copolymerized using a catalyst system comprising a solid compound comprising titanium trichloride and an organometal compound in the absence of a liquid diluent and having a specific sequence distribution, but the low-temperature heat sealing property of the copolymer is insufficient and also there are problems that the blocking resistance is poor and the transparency deteriorates with the passage of time.
Furthermore, JP-A-60-16645 discloses a propylene-butene-1 copolymer obtained in a vapor phase in substantially the absence of a liquid medium, the copolymer showing good results in the low-temperature heat sealing property, the transparency, and the blocking resistance. However, according to the present inventors' duplication of the JP-A, the hot tack property was insufficient.
A propylene-ethylene-butene-1 terpolymer obtained by copolymerizing propylene, ethylene, and butene-1 is further well known as a heat sealing resin.
For example, JP-A-54-26891 describes a production method of an olefin copolymer by supplying from 0.1 to 4% by weight of ethylene and from 1 to 30% weight of an .alpha.-olefin having from 4 to 8 carbon atoms to propylene in a polymerization system. JP-A-53-26882 also describes a propylene terpolymer containing from 0.5 to 1.9% by weight of ethylene and from 0.5 to 4.9% by weight of butene-1 and having a substantially statistic comonomer distribution, and a production method of the propylene terpolymer. Further, JP-A-55-115416 describes that a low-crystalline soft or semi-soft copolymer can be obtained by copolymerizing propylene with from 0.2 to 9 mol% of ethylene and from 0.2 to 9 mol% of a straight chain .alpha.-olefin having at least 4 carbon atoms.
However, the copolymers and terpolymers described above do not simultaneously satisfy the low-temperature heat sealing property and the hot tack property, and also do not show an excellent transparency, an excellent blocking resistance, and an excellent solvent resistance.